A 20-pound toddler traveling at 30 MPH will require 600 lbs of restraining force to keep him or her in place.

Children should be in rear-facing car seats at least until age two. This is the consensus of the American Academy of Pediatrics based on decades of research that have consistently shown that small children are safer riding backwards. But now, some parents are insisting on keeping their kids rear-facing well after their second birthdays. Is this an overreaction or a new best practice? The best way to understand the issue is by looking at the physics of collisions.

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Why You Want To Be Riding Backwards

Only 7 percent of fatal car crashes involve rear impacts, and the vast majority (60-80 percent) of crashes that kill come from the front or side of the vehicle. That’s a key statistic because, in a front-end collision, the driver snaps forward toward the point of impact (and is hopefully restrained by a seatbelt), while a rear-facing child falls backward (right into the car seat).

Since most crashes occur in this fashion, keeping your child rear facing stacks the deck in your favor. Odds are that, when a serious crash occurs, your rear-facing child won’t snap forward into a harness and will instead experience crash forces that are about equally diffused along the car seat. This helps keep the neck and spine just where we want them (more on that later).

The Physics of a Crash

Why is it so important for the crash forces to be evenly distributed into the seat, rather than unevenly across a harness? Because every crash actually involves three collisions: the vehicle striking an object, the body of the occupant striking a harness, and the occupant’s internal organs striking the inside of the body. In a front-end collision, the child is already seated (and therefore isn’t traveling into the car seat so much as being pushed further into it), so the second and third collisions occur more or less at the same time, reducing the odds of organ damage.

These forces, by the way, aren’t trivial. A 20-pound toddler traveling at 30 MPH will require 600 lbs of restraining force to keep him or her in place. In the rare event of a serious rear collision, that means a rear-facing child snaps forward at 30 MPH and feels 600 lbs pushing back. But in the more likely scenario of a front-end collision, those 600 lbs will push the child back into the seat.

Toddler Bones are Best Seated in Reverse

Let’s say you buck the system and put your 1-year-old in a front-facing car seat. In the event of a front-end collision, we’re talking hundreds of pounds of stopping force (the exact figure is simply your child’s weight multiplied by the speed of the vehicle) exerted on the harness as your kid snaps forward. What does that mean for your child’s developing body?

Well, it’s bad news for the spinal cord. Small children have vertebrae that consist of small bits of cartilage, which will fuse into bone over time. Before age two, there’s only a 50 percent chance that the vertebrae in the neck have finished converting from cartilage to bone, and in most cases a child won’t sport a fully reinforced spine until age six. Cartilage, unlike bone, allows the spinal cord to stretch up to two inches. Yet, it only takes ¼ of an inch to cause paralysis or death.

When an adult jerks forward suddenly, his or her bony vertebrae usually prevent the spinal cord from stretching too much (unless the bone itself breaks). But when a toddler experiences those same forces, the spinal cord is wont to stretch beyond the point of no return. It also doesn’t help that toddler spines support heavy heads (up to 25 percent of the child’s total body weight) so, when your kid flies into a harness, that cranial cannonball forces the spine to stretch even more.

In other words, a harness protects your child’s torso but does little to prevent his or her spinal cord from snapping in the event of a high-speed crash. The implications are counterintuitive: You want to position your car seat so that the harness is least likely to be what stops your child from flying toward the point of impact. Crash statistics tell us that the best way to accomplish that is by keeping your kid facing backwards.

How Long Should My Kid Ride in Reverse?

Ideally? Until your child’s spinal cord ossifies. But without a CT scan, you’re not going to know when that happens (and even at age three, the odds barely approach 50 percent).

So when parents choose to keep their kids in rear-facing car seats past their second birthdays, they’re not overreacting. Most modern car seats can hold three-year-olds quite comfortably and, until those vertebrae turn to bone, having your kid ride backwards actually might be the best advice. There are certainly no additional risks (as long as your child doesn’t exceed the manufacturer’s height and weight requirements) and a lot of potential gain.

“Rear facing is not a choice to be made based on parenting style or opinion; it’s one based on scientific fact,” according to the non-profit Car Seat For The Littles. “The more we know about physics and physiology, the better we’re able to protect our kids from severe injury.”